The semiconductor industry has been using single substrate (silicon wafer) processing chambers for some time because the chamber volume can be minimized, contamination of the substrate has been reduced, process control is increased and, consequently, yields are improved. Further, vacuum systems have been developed, such as described in Maydan et al, U.S. Pat. No. 4,951,601, that allow several sequential processing steps to be carried out in a plurality of vacuum processing chambers connected to a central transfer chamber, so that several processing steps can be performed on a substrate without its leaving a vacuum environment. This further reduces the possibility of contamination of the substrates.
Recently the interest in providing large glass substrates with up to one million active thin film transistors thereon for applications such as active matrix TV and computer displays has been heightened. These large glass substrates, generally of a size up to about 350.times.450.times.1 mm, require vacuum processing chambers for deposition of thin films thereon. The basic methods and processing chambers, e.g., plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), etch chambers and the like, are similar to those used for depositing layers and patterning thin films on silicon wafers. A practicable system that can perform multiple process steps on glass substrates is disclosed by Turner et al in copending U.S. patent application Ser. No. 010,684, filed concurrently herewith entitled "VACUUM PROCESSING APPARATUS HAVING IMPROVED THROUGHPUT".
In conventional single substrate PECVD chambers, the substrate to be processed is supported by a heated susceptor and reaction gases are fed to the chamber via a gas dispersion plate mounted above and generally parallel to the substrate. The gas dispersion plate and susceptor are connected across an RF source and, when power and reactant or precursor gases are turned on, a plasma from the precursor gases forms in the region between the gas dispersion plate and the substrate. By manipulation of gas flows, temperatures, and pressure in the chamber maintained by a vacuum exhaust system, most of the particulates in the plasma region can be carried by the gases away from the susceptor and substrate and to the exhaust system. However, particulates remain a problem and many modifications of equipment have been made to reduce this problem. Nevertheless, the vacuum chambers must be cleaned periodically to remove particulates or solid material that has deposited on the walls, gas dispersion plate and other components conventionally employed in vacuum processing chambers.
In the case of large glass substrates, this problem is exacerbated because, in order to have a minimum chamber volume, the area between the walls and the substrate is quite small. At the same time, in order to achieve good film thickness uniformity as well as other film properties, it is necessary to provide a gas exhaust geometry in the chamber such that the rate at which the gas exits the substrate deposition region of the chamber is substantially uniform everywhere along the periphery of the substrate. Further, in the vacuum system described by Turner et al, referred to hereinabove, a plurality of processing chambers can be connected to a transfer chamber. In order to be accessible to a robot in an adjacent transfer chamber, an entry port for the large glass substrate must be located on one side of the chamber. It is also very important that particulates not collect near the area of the entry port so that particulates are not deposited directly onto substrates as they are brought in and out of the processing chamber, nor are they exported to the transfer chamber because this would obviously lead to the eventual contamination of the whole system.
Thus a need to improve the exhaust system of a single substrate vacuum chamber for processing large glass substrates was manifest, and the prompt removal of particulates in the processing chamber is absolutely necessary.